Why are performing JavaScript functions slower than doing the same thing manually?

Question

An example of what I'm talking about.

This implementation, is much faster..

var data = [1, 2, 3, 4];
var currIndex = 0;
var copied = [];
for (var i = 0; i < data.length; i++) {
  copied[currIndex] = data[i];
  currIndex++;
}

Than this:

var data = [1, 2, 3, 4];
var copied = [];
for (var i = 0; i < data.length; i++) {
  copied.push(data[i]);
}

It's obvious that when writing JS, most people would write the second implementation as it's clearer what they're doing. Plus, with the first one, you are re-inventing the wheel.

Not sure why JS functions are so expensive.

Define "much faster". "why JS functions are so expensive" --- *expensive* compared to *what*? — zerkms, Aug 19 '16 at 02:47
Related: [Why is `array.push` sometimes faster than `array[n] = value?`](http://stackoverflow.com/q/614126/4642212) and [Is there a reason JavaScript developers don’t use `Array.push()`?](http://stackoverflow.com/q/15649899/4642212). — Sebastian Simon, Aug 19 '16 at 02:49
Inlining an operation is often faster than calling a function that does that operation. `copied[currIndex] = data[i];` is inlining the operation rather than calling a function. Same in lots of languages. Plus, `.push()` may have some error checking on it that you do not. You would expect `obj.i++` to be faster than `obj.increment(i)` right? — jfriend00, Aug 19 '16 at 03:01
Is your question "why are functions slower", or "why do people use functions even though they're slower"? — , Aug 19 '16 at 03:21
Related: [Javascript for..in vs for loop performance](http://stackoverflow.com/questions/13645890/javascript-for-in-vs-for-loop-performance) and: [Why is using “for…in” with array iteration a bad idea](http://stackoverflow.com/questions/500504/why-is-using-for-in-with-array-iteration-a-bad-idea) — Mehdi Dehghani, Aug 19 '16 at 03:22

Why are performing JavaScript functions slower than doing the same thing manually?

0 Answers0